Many types of machines may be driven by electric motors. In some cases, the machine can encounter undesirable operating conditions while the motor continues turning as though nothing had happened. While these undesirable conditions, such as a broken connection or overloading of the machine may be readily detectable visually or aurally, this requires the presence of an operator or supervisor and is undesirable when it is desired to have a largely automated plant with many machines controlled or supervised from a central console.
The problem may perhaps be better appreciated if a specific example is considered and a good example is a rod mill. A rod mill comprises a large, horizontally-disposed, cylindrical drum which contains numerous steel rods extending lengthwise of the drum. The drum may be approximately half-full of these rods. Into one end of the drum there is introduced crushed ore (to be further crushed), water, and possibly "fines", i.e., finely divided ore material. The drum is continuously rotated by a large electric motor and the inside of the drum is normally corrugated. As the drum rotates, the rods tend to ride up with the drum but eventually, during normal operation, they will roll or slide down. This action crushes the ore which eventually leaves the other end of the drum in a continuous process. Now it can be appreciated that if the mill is lightly loaded, the mass of rods will only "ride up" a relatively small amount but with the addition of more ore, they will tend to ride up further. If too much ore is introduced, the rods may pass the topmost position and tumble back down getting entangled with the other rods. This is called a "tangle" and is a very undesirable situation as it can require several hours or even several shifts to untangle the rod mill. On the other hand, it is desirable to operate the mill at high capacity and this can be done if the mill is fed with material at the maximum rate which is just short of that which will cause a tangle. It should also be noted that a change in hardness of the ore material can affect the operation of the mill even if there is no change in the rate of feed to it. If the ore material were suddenly to become harder, tangling might result.
It would be highly desirable to remove the conditions which cause the tangle. In fact, a trained operator can tell just by listening when tangling is about to occur and can take corrective measures. Attempts have been made to detect incipient tangling by placing a microphone near the mill and feeding the output of the microphone through an amplifier and filter arrangement to detect the sound characteristic of a rod mill approaching a tangling condition. However, these measures have proved very unreliable, perhaps because of the amplitude and variety of noises encountered around the mill. Attempts have also been made to detect incipient tangling by measuring the average power drawn by the mill motor but these have also been unsuccessful as the variations in power as opposed to changes in frequencies are not very substantial and do not differentiate between incipient tangling and load changing.
It has been found, and the present invention is predicated on the discovery, that the current to the mill motor, normally 60 Hz, has superimposed on it frequencies of significant amplitude in a band of frequencies characteristic of predetermined operating conditions, in this case, incipient tangling. These frequencies would depend on the particular machine. Also, the frequencies superimposed depend on the particular event. In other words, for a particular machine, an incipient tangling superimposes a character band of frequencies on the motor current. This characteristic band of frequencies disappears at the point of actual tangle. A different condition, e.g., a faulty bearing would superimpose another band of frequencies. The event need not be an abnormal operation of the machine. Whatever the event may be, so long as it causes this characteristic change in harmonics of the motor signal, the superimposed frequencies can be identified. It is the occurrence of such characteristic frequencies that is utilized in the present invention. It should be noted that the signal that relates to the event is different from the power signal related to the mechanical load. In the instance of the incipient rod tangle, as indicated above, variations in the power level are not very substantial as opposed to changes in characteristic frequencies.